Light signals in railway signalling systems must be operated on a fail-safe basis, i.e., their operation must be continuously monitored and any failure must be immediately detected and rendered ineffective by putting into operation a substitute signal, such as a secondary filament.
It is known (see, for example, "Eisenbahntechnische Praxis", 1959, No. 3, pp. 25 and 26) to operate each signal lamp via a separate lamp transformer which is located near the signal lamp and causes the supply circuit from the interlocking station to the signal control unit, where the circuit goes through the primary winding of the lamp transformer, to be not interrupted if the signal lamp fails due to a filament break. The lack of load on the secondary side of the lamp transformer only results in a reduction of the current in the supply circuit. This decrease of current can be detected and indicated by means of a monitoring facility consisting of a monitoring transformer having its primary winding included in the supply circuit and a monitoring relay connected to the secondary winding of the transformer. As stated in the article referred to above, the components required to operate the signal lamps must be precisely matched to be able to operate on a fail-safe basis. In addition, the supply voltages for the signal lamps must be accurately adjusted to the respective control distance, particularly if the signals are to be supplied at night with a lower voltage then by day. The control distance is limited to a value (6.5 km) which appears too small for large interlocking plants as are desirable today.